Wednesday, September 26, 2012

Pyrometer is a device
which uses the relationship between the electromagnetic radiation emitted by a
body and the temperature of this body. In order to better understand the
phenomenon which forms the basis of pyrometry, it is useful to explain the
concept of the blackbody, and the
differences between it and real objects.

The term blackbody is ideal, and designates a
body which radiates more electromagnetic
energy for all wavelengths intervals than any other body of the same area and
at the same temperature, and absorbs all the radiation it intercepts. Fig.1 presents one of the classical
blackbody model.

Figure 1. A classical blackbody model.

The temperature of the
blackbody determines the nature and extent of such radiation. Stefan-Boltzmann’s law says, that

the blackbody with a finite absolute temperature (T)
emits radiant (ie, in all directions) electromagnetic radiation (EMR) per unit
area of this blackbody and per second with intensity which is proportional to T4,
according to an equation:

ET = σ * T4, (2)

where,

ET- total EM energy
emitted by the blackbody in all directions per unit area (1.m2) and per unit time
(1,s),W/m2;

σ-
Stefan-Boltzmann’s constant, equal to 5.67051*10-8, W/(m2*K4);

T - an absolute temperature of the blackbody, K.

Fig. 2 shows the relationship between EMR emitted by a perfect blackbody as a
function of temperature. The area under these curves is equal to the total
energy (emitted by a black body) per second per unit area. This body at low
temperatures emits EMR in the region of long wavelengths. This region spreads
from far-infrared to microwave region (5 mm < λ < 100 mm, where λ is the
wavelength in mm, 10-6
m). With increasing the blackbody temperature, the emission peaks move into the
region of shorter wavelengths. At very high temperatures the blackbody emits in
the near visible wavelengths region. Visible region corresponds to the
wavelengths from 0.7 mm (red) through
0.62 mm (orange), 0.58 mm (yellow), 0.53 mm (green), 0.47 mm (blue) to 0.42 mm (violet).

Real
objects emit and absorb less EMR than blackbodies, and this difference is
dependent on the wavelength, so nonblackbodies can not exactly follow
relationship shown in Fig. 2. For
this purposes corrections should be used, otherwise, the apparent temperature
will be lower that the actual temperature. Also, it is necessary to take into
account the loss of emitted radiation when it passes through the media between
the emitting body and a measuring instrument.

Figure 2. EM radiation emitted by the blackbody at various temperatures.

There are two types of
pyrometers: optical (monochromatic or
narrowband) and radiation (total
radiation or broadband) pyrometers. The last devices originally were called
radiation pyrometers, then radiation thermometers, and more recently infrared
thermometers. However, the first their name (radiation pyrometers) is still
widely used at present. These devices have high accuracy of ±0.01 °C as a standard instruments,
and from ±0.5 to ±1% for industrial purposes.

a).Optical pyrometers, sometimes referred
to as brightness thermometers,
generally involve wavelengths only in the visible part of the spectrum. When
the temperature of the body increases, so does the intensity at any particular
wavelength. If two bodies have the same temperature, then intensities of those
two objects are equal. In this type of a pyrometer the intensity of a certain
wavelength of a heated body is compared with that of a heated platinum filament
of a lamp (see Fig. 3 ).

Figure
3. An optical pyrometer.

An object 1
which temperature is to be measured, emits electromagnetic radiation with
intensity proportional to its absolute temperature. This radiation passes
through lens 2 and red optical filter 3. Optical filter picks out only the
desired wavelength - red. Then radiation focuses on the platinum filament of a
lamp 4,
and passes through another filter 5, lens 6, viewing system 7.
The viewer 8 sees the platinum filament superimposed on an image of the
object 1. When the temperature of the filament is low comparing with
that of the object, the viewer sees the filament as a dark line on the bright
background image of the object. The lamp 4 is connected in series with an
electrical battery 9, a variable resistor 10 and an ampermeter 11.
By reducing the resistance of the resistor an electrical current passing
through the filament increases. So does the temperature of the filament and its
brightness. For a certain value of an electrical current (corresponded to a
certain value of an object temperature), the brightness of the platinum
filament will match the brightness of the object 1. At this setting the
viewer cannot distinguish between the image of the object and the filament. At
this time the measurement of temperature is performed. The scale of the
ampermeter is calibrated in the units of temperature.

The lower temperature limit
for optical pyrometers is determined by the temperature at which objects become
visible in red (about 225 °C). However,
there are devices which are able to measure even lower temperatures down to -50
°C. The upper
limit varies from 600 to 3000 °C, and is limited
by the melting point of the platinum filament. An accuracy is typically varied
from ±5 to ±10 K.

b).Radiation pyrometers, being very simple
and cheap, use an exponential relationship between a total emitted EMR energy
and given temperature. In radiation pyrometers (see Fig. 4) EMR energy emitted at infrared (2.5 < l < 20 mm) to visible wavelengths
(0.42 < l < 0.7 mm) from an object 1
is focused by a spherical reflector 2 on a series of micro-thermocouples
attached to a blackened platinum disc 3. The radiation is absorbed by the
disc, which temperature is increased, so does thermal electromotive force U
developed by the series of thermocouples. This thermal electromotive force is
proportional to the temperature of hot junctions of thermocouples, and,
finally, to the temperature of the object 1. The advantage of these pyrometers
is that their operation slightly depends on the wavelength.

Figure 4. A total radiation pyrometer

The lower limits for
radiation pyrometers vary from 0 to 600 °C, the upper limits vary
from 1000 to 1900 °C. The accuracy
varies from ±0.5 to ±5 K, depending on cost. They
are widely used for temperature measurements in metal production facilities,
glass industries, semiconductor processes, etc.

Article Source:: Dr. Alexander Badalyan, University of South Australia

If semiconductors or
heat-treated metallic oxides (oxides of cobalt, copper, iron, tin, titanium,
etc.) are used as the materials for producing temperature sensitive elements,
then these temperature transducers are called thermistors (the name is derived from the term of ‘thermally
sensitive resistor’). These oxides are compressed into the desired shape from
the specially formulated powder. After that, the oxides are heat-treated to
recrystallise them. As the result of this treatment the ceramic body becomes
dense. The leadwires are then attached to this sensor for maintaining
electrical contact.The following relationship
applies to most thermistors:

Fig. 3.15 shows relationship
between temperature and resistance for a thermistor.Thermistors have negative
thermal coefficient of electrical resistance. It means that when temperature
increases the electrical resistance of thermistor decreases. They have greater
resistance change (this is an advantage) compared with RTD in a given
temperature range. For example, if we compare what change in resistance will be
caused by variation of temperature in 1 °C for Platinum and Copper
RTD and for
thermistor (see Fig. 2) in the
temperature range from 273.15 to 423.15 K (ie, from 0 to 150 °C), we will obtain the
following values:

• for platinum RTD - 0.38, Ohm / ̊C;

• for copper RTD - 0.04, Ohm / ̊C;

• for thermistor - 0.65, Ohm / ̊C;

Figure 2. Thermistor resistance vs temperature curve.

Wheatstone bridge and
resistance measuring constant current circuits, similar to that used in the
case of RTDs, are used for resistance measurement of thermistors. Despite their high
sensitivity, thermistors have a worse accuracy and repeatability (this is the
disadvantage) comparing with metallic RTDs. Since the resistance vs temperature
function for thermistors is non-linear (although, some modern thermistors have
a nearly linear relationship of temperature
vs resistance), it is necessary to use prelinearisation circuits before
interacting with related system instrumentation. In addition, due to the
negative thermal coefficient of electrical resistance an inversion of the
signal to positive form is required when interfacing with some analog or
digital instrumentation. Therefore, thermistors are not widely used in process
instrumentation field, at least at present. However, they have been well
accepted in the food transportation industry, because they are small, portable
and convenient. Another field of their growing application are heating and
air-conditioning systems, where thermistors are used for checking the
temperature in flow and return pipes.

All the discussed above
instrumentation for temperature measurement refers to contact-type devices, because their
sensitive elements are immersed in the measuring media. When dealing with
temperatures above 1500 °C, contact-type
temperature measuring devices are not applicable, because irreversible changes
occur in metals which form their sensitive elements. It is possible to perform non-contact measurement of temperature
by optoelectronic transducers.

Article Source:: Dr. Alexander Badalyan, University of South Australia

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